At this point it can be clearly seen the dependence
upon mass of the dipole moment derivative,since 
is a
normal coordinate. Recall that the normal coordinate is a
orthogonalized mass-weighted Cartesian coordinate so different
isotopomers will have different normal coordinates.
Pictorially, look at Figure 4. Here is a plot of dipole moment
s for a system with
only two normal coordinates, 
and 
. For a given displacement
of and , the dipole moment was plotted. The components
of the dipole moments along each coordinate has also been projected.
The gradient of the dipole moment with respect to a normal coordinate
will be greatest for a component that is perpendicular to the curves.
Likewise, if the component should run long a curve, the gradient
will be very small. As such, component 
will have a more intense
transition than component 
. Now, for an isotopically
substituted system, represented by 
and 
, the coordinate
system has rotated with respect to the dipole and the new components
are now much more similar in intensity since their gradients are almost
identical.